The present invention relates to an innovatory method and a plant applying such a method, for continuously producing a bituminous conglomerate in which exact proportions between the mixture components are adopted.
Known in the art of producing bituminous conglomerates is the importance of respecting precise xe2x80x9crecipesxe2x80x9d in the conglomerate composition. Tolerances in the recipes are usually limited; for instance, the bitumen percentage for each specific recipe is established with tolerances in the order of half a point percent and said tolerances for particular recipes can even be included between 0.05% and 0.08%.
By way of example, a typical recipe can be:
Unfortunately, aggregates are usually wet, having a moisture content generally included between 3% and 7%. For instance, typical humidity characteristics of the aggregates on the stocks can be:
The moisture content in the aggregates as well as the filler content can vary depending both on the particle sizes and, more simply, on the they position take in the storage heap. In addition, it may easily happen that the moisture degree, in the same yard and for the same material, can vary even of some points percent, during the production of one and the same product.
The different and variable moisture percentages of the different components give rise, as a consequence, to calculation mistakes as regards the bitumen amount to be introduced into the mixture.
In order to solve this problem, use of an aggregate dryer as a unit separated from the mixer has been proposed, so as to eliminate humidity before picking up the established aggregate amounts as stated by the particular recipe.
The above introduces a complication in the plant and often there is a preference for plants in which the whole production process takes place within the drying oven to which both bitumen and damp aggregates are conveyed to be mixed together. In fact, these plants (named xe2x80x9cDrum Dryerxe2x80x9d, xe2x80x9cDrum Mixerxe2x80x9d, xe2x80x9cCEMxe2x80x9d plants, etc.) have the advantage that their purchase and use is not expensive. On the other hand, aggregates are necessarily weighed when wet (usually by a weighing conveyor belt) and, even if an attempt is made to carry out a compensation of the amounts by deducting the moisture percentage detected on a sample in a laboratory, the bitumen amount to be admitted to the mixture is only theoretically calculated and this leads to a non-eliminable and often high variability in the recipe as really applied. In addition to the above, the natural filler amount present in the aggregates and resulting from the crushing process can very widely vary in percentages, ranging from 2-3% to 20-25%, from yard to yard but also in the same yard (for instance, when the aggregates do not come from the same source and/or are produced with ores of different qualities and/or qualities varying in time). This variability depends on the ore nature, since the ore can be more or less hard and friable, and also on the type of crusher used and, above all, on the presence or not of the step of washing the aggregates with water after crushing. Carrying out washing of the aggregates or not, so as to practically eliminate all excess filler, depends on technical or economical choices (if lack of water exists or the water supply is difficult, for example).
Since the filler amount required in the recipe is precise, when the filler amount present in the aggregates is too much, part of it will have to be necessarily discarded. Usually the filler carried along by the gases coming out of the oven reaches an appropriate filter. This therefore takes place downstream of weighing, which operation is carried out on the conveyor belt. At all events, the recovered filler is introduced into the mixing region by means of metering screws having a controlled number of revolutions to carry out a volumetric dosage. As a result, if there is an excess amount of filler, which happens very often, no control exists as regards the discarded amount and the statistical determination of this filler amount relies on laboratory tests.
In conclusion, the evaluation errors on the exact amount of bitumen to be introduced into the oven depend on the above estimate that, although precise, introduces an uncertainty degree into the recipe.
It is a general object of the present invention to obviate the above mentioned drawbacks by providing a production method and plant enabling a bituminous conglomerate to be continuously produced, with high precision and constancy in respecting the xe2x80x9crecipexe2x80x9d.
In view of this aim, in accordance with the invention a method has been devised for production, in a continuous plant, of a bituminous conglomerate containing aggregates and at least bitumen, which method comprises the steps of drying the aggregates and mixing them with a measured amount of incoming bitumen to form the bituminous conglomerate; weighing the obtained bituminous conglomerate; calculating the amount of dry aggregates present in the conglomerate as the difference between the detected weight of the obtained bituminous conglomerate and said measured amount of bitumen; adjusting in real time at least the amount of incoming bitumen depending on the calculated amount of dry aggregates, so as to keep this bitumen amount within preestablished proportions with respect to the calculated amount of dry aggregates.
Still in accordance with the present invention, a continuous plant for producing bituminous conglomerate has been also devised which comprises a drier-mixer unit to which feed means sends aggregates and at least bitumen so as to form the bituminous conglomerate, characterised in that it comprises means for measuring the amounts of bitumen sent to the drier-mixer unit, means for weighing the bituminous conglomerate coming out of the unit, and a control system calculating the amount of dry aggregates present in the outgoing conglomerate as the difference between the weight of the bituminous conglomerate detected by the weighing means and the amounts of bitumen detected by the means for measuring said amounts, the control system operating the bitumen feed means so as to adjust in real time at least the amount of bitumen entering the unit, depending on the calculated amount of dry aggregates, so as to keep this bitumen amount within preestablished proportions with respect to the calculated amount of dry aggregates.